Various portable systems for medical use are known which employ a battery for providing power for the system. For example, a battery may be used to power an ultraviolet light source for sterilization purposes. For such usage, it is essential that the battery be capable of delivering sufficient energy for the sterilization cycle.
We have discovered a circuit for providing an effective system whereby the battery may be properly maintained for delivering sufficient energy in medical environments. To this end, a battery charger which provides a constant current charge to the battery is utilized in contrast to a constant voltage charger. A constant current charge is provided to the battery until the battery voltage reaches a threshold and, when the battery voltage is above the threshold a trickle charge having a much lower amount of current is applied to the battery. The trickle charge is sufficient to complete the charge to the battery and to compensate for the self-discharge of the battery.
Battery chargers are known in the prior art for providing a constant current charge to the battery until the battery voltage reaches a threshold, and for providing a trickle charge thereafter. In Coleman, et al. U.S. Pat. No. 3,919,618, this basic type of battery charging is disclosed. However, Coleman, et al. discloses the use of a voltage reference element diode for providing temperature compensation and also for developing a hysteresis loop. The Coleman, et al. circuit is such that in the high rate charging mode there is less current passed through the voltage reference element diode than in the low rate charging mode. This is essential to the Coleman, et al. method for developing the hysteresis loop. An undesirable side effect of this is that the temperature coefficient exhibited by this voltage reference element diode varies with the change in current. Thus the temperature compensation provided by the Coleman, et al. circuit is compromised.
For certain medical uses the accuracy and efficiency of a battery charger cannot be compromised. We have discovered a constant current type battery charger in which there is no compromising of the temperature compensation in order to provide hysteresis. In the illustrative embodiment of our invention, the temperature compensation is independent of whether the charging is in the high rate mode or the low rate mode. In addition, in the illustrative embodiment of our invention the mechanism which introdudes hysteresis into the voltage reference is also independent from temperature effects.
In addition, we have discovered that in medical usage it is appropriate that the state of the battery condition be checked. To this end, in the illustrative embodiment of our invention a circuit is provided for testing the state of the battery condition.